Locking devices for rollers of architectural coverings are well known. One such device capable of locking the roller of a window shade in different positions is described in EP 0 087 146. The device has a coil spring or wrap spring brake and a cam sleeve with a bifurcated cam groove engaged by a cam pin, connected to the roller. When the roller rotates, the cam pin moves along the cam groove and pushes the cam sleeve to loosen the coil spring brake when the shade is being pulled down and when it is released after being pulled down in order to roll it up. When the shade is pulled down from its uppermost position and than released, the coil spring brake is tightened and locks the roller.
Another locking device, described in EP 0 356 403, has two sets of corresponding opposed teeth and a cam sleeve with a bifurcated cam groove engaged by a cam pin connected to the roller of a window shade. When the roller rotates, the cam pin moves along the cam groove and pushes one set of teeth to engage the other set of teeth when the shade is released at a given position, after being pulled down. The shade is than locked at that position. The cam pin moving along the cam groove drives one set of teeth either into or out of engagement with the other set of teeth as required for locking or for raising or lowering the shade.
Yet another locking device, described in U.S. Pat. No. 4,662,423, uses a changeover wheel with an axially-projecting pin and a guide wheel with a guideway, including guide portions made of a leaf spring. The free end of the pin moves along the guide path of the changeover wheel. The pin is biased radially outwardly by a coil spring, but this bias can be overridden by the leaf spring guide portions which will move the pin radially inwardly, against the bias of the coil spring. The guide wheel is connected to the roller of a window shade and rotates when the shade is raised or lowered, but a coil spring brake allows the changeover wheel only to rotate when the shade is lowered. In this regard, when the shade is lowered, the roller and the guide wheel rotate, and the guideway moves the pin and thus the changeover wheel to a first rest position. This connects the changeover wheel to the rotating guide wheel, causing it also to rotate. The rotation of the roller also loosens the coil spring, so that the changeover wheel is free to rotate. When the shade is released after being lowered so that it starts moving up, the guide wheel is rotated so that the pin moves from the first rest position to a second rest position in the guideway. As result, the coil spring is tightened, so that the changeover wheel cannot rotate. Since the changeover wheel is still connected to the guide wheel by the pin and the guide wheel is connected to the roller, all rotation is stopped by the tightening of the coil spring. To raise the shade, it is first pulled down slightly, causing the pin to move from the second rest position and to be free to move radially outwardly under the bias of the coil spring, thereby freeing the guide wheel to rotate. Since the guide wheel is connected to the roller of the shade, both now rotate and the shade is raised.
These locking devices depend on coil springs or sets of interengaging teeth to stop rotation of a roller. Such devices tend, therefore, to be rather bulky and to take up valuable space in the hollow tubular roller, on which a shade is wound. Also such devices tend to be rather complicated and time consuming to assemble, especially if the faulty assembly of such devices is to be minimized.